This invention relates generally to gas turbine engines, and more particularly, to methods and apparatus for controlling the operation of gas turbine engines.
Gas turbine engines typically include a compressor section, a combustor section, and at least one turbine section. The compressor compresses air, which is mixed with fuel and channeled to the combustor. The mixture is then ignited generating hot combustion gases. The combustion gases are channeled to the turbine, which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to power a load, such as an electrical generator, or to propel an aircraft in flight.
Gas turbine engines operate in many different operating conditions, and combustor performance facilitates engine operation over a wide range of engine operating conditions. Controlling combustor performance may be used to improve overall gas turbine engine operations. More specifically, permitting a larger variation in gas fuel composition, for example, heating value and specific gravity, while maintaining NOx emissions and combustion dynamics levels within predetermined limits. Gas turbines equipped with Dry Low NOx (DLN) combustion systems typically utilize fuel delivery systems that include multi-nozzle, premixed combustors. DLN combustor designs utilize lean premixed combustion to achieve low NOx emissions without using diluents such as water or steam. Lean premixed combustion involves premixing the fuel and air upstream of the combustor flame zone and operation near the lean flammability limit of the fuel to keep peak flame temperatures and NOx production low. To deal with the stability issues inherent in lean premixed combustion and the wide fuel-to-air ratio range that occurs across the gas turbine operating range, DLN combustors typically have multiple fuel nozzles in each combustion chamber that are fueled individually or in sub-groups. The gas turbine fuel system has a separately controlled delivery circuit to supply each group of nozzles in each chamber. The control system varies the fuel flow (fuel split) to each circuit over the turbine operating range to maintain flame stability, low emissions, and acceptable combustor life. Fuel flow to each nozzle sub-group is controlled via a gas control valve (GCV). The fuel split acts to divide the total fuel command (Fuel Stroke Reference) amongst the active GCV's, and the resulting percentage GCV fuel flow command is converted to a valve position to achieve the desired fuel flow to the nozzle sub-group.
To convert the percentage GCV flow command to valve position, a gas fuel system flow gain in terms of valve flow capacity coefficient, Cg is determined. The valve capacity coefficient is translated to valve position using the known valve flow characteristic. This allows the use of multiple valves with varying capacities. The flow gain, also called GCV flow scalar, is based on the maximum required Cg during the maximum fuel flow operating condition.
The inputs used to calculate the flow gain are dependant on fuel constituents and the application of this flow conversion technique is limited to applications with fairly constant fuel properties. Traditional methods using the flow gain assume the fuel properties are constant throughout the loading range, which is not always the case. Therefore, without correcting for changes in fuel properties the flow gain will not properly linearize the flow command across the loading range. This can lead to an undesirable droop non-linearity and can cause load transients when fuel properties change significantly, for example during fuel transfers or following large changes in fuel temperature.
Prior techniques have used biases to the flow gain where an actual to design gas temperature was used to bias the flow gain, however this type of correction generally accounts for fuel temperature, and fuel composition is assumed to be relatively constant. However, when power augmentation systems are used, like a fuel moisturization system, the magnitude of the changes in fuel physical properties is significant, and a new technique is needed to accurately calculate the correct flow gain.